This invention relates to the amplitude stabilization of continuous-wave (CW) lasers and, in particular, to the stabilization of intrinsically amplitude-unstable (IAU) lasers to reduce laser intensity fluctuations.
Lasers with low relative intensity noise (low RIN) are important in many applications, such as optical communications, testing and measurement. Amplitude variations in these systems will degrade performance, leading to, for example, to an increase in bit error rate and loss of measurement resolution. In optical communication systems, for example, data is sent as a modulation of an optical carrier signal from a laser source. The quality of the data transmission is contingent upon the quality of the carrier signal, and any amplitude fluctuation in the laser directly degrades the integrity of the data signal. It is important in an optical system that any laser used as a signal source be substantially free of excessive intensity fluctuations.
Unfortunately, certain laser systems that may otherwise possess desirable characteristics cannot be used for low RIN applications because the lasers are intrinsically amplitude unstable (IAU). An IAU laser is one in which the natural lifetime of the upper lasing state (excited state) is substantially longer than the photon lifetime in the cavity. IAU lasers are subject to spiking, intensity fluctuations, and relaxation oscillations, making them unsuitable for applications where low RIN is required.
Therefore there is a need for stabilization of IAU lasers to permit their use in low RIN applications.
In accordance with the invention, the IAU laser is stabilized in amplitude by providing a multiphoton absorbing medium disposed in the cavity to reduce intensity fluctuations. A pump source to excite the gain medium is coupled to the cavity. In operation, the multiphoton absorbing material absorbs primarily at high intensity levels, selectively increasing the loss at high intensities without correspondingly increasing the loss at low intensities. In an advantageous embodiment, the active medium comprises erbium-doped glass and the multiphoton absorber comprises a body of semiconductor exhibiting two-photon absorption at the laser emission wavelength.